Dispersing sodium within molten aluminum

ABSTRACT

A METHOD OF DISPERSING SODIUM INTO MOLTEN ALUMINUM. A SODIUM-CONTAINING, ESSENTIALLY INSOLUBLE HEAVY METAL IS INTRODUCED INTO MOLTEN ALUMINUM AT A TEMPERATURE EXCEEDING THE MELTING TEMPERATURE OF THE SODIUM-CONTAINING HEAVY METAL. THE SODIUM DIFFUSES FROM THE HEAVY METAL INTO THE ALUMINUM TO MAINTAIN A DESIRED LEVEL OF SODIUM IN THE ALUMINUM FOR AN EXTENDED PERIOD. A SOLIDIFIED SHAPED ARTICLE HAVING A MODIFIED SILICON STRUCTURE CAN BE CAST FROM THE SODIUMIZED ALUMINUM.

United States Patent O T 3,705,029 DISPERSING SODIUM WITHIN MOLTEN ALUMINUM George S. Foerster, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich. No Drawing. Filed Jan. 6, 1971, Ser. No. 104,475

Int. Cl. C22c 1/02 US. Cl. 75-138 11 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to a process for the addition of sodium to molten metals. The invention also concerns the sodium modification of silicon phases in aluminum base alloys.

It is known that modification of silicon in aluminum alloys generally improves the casting characteristics, mechanical properties, and machineability of the metal. To modify the silicon particles in the aluminum alloy, metallic sodium is frequently added to the molten aluminum base metal. Sodium has a melting point of approximately 207 F. and naturally melts rapidly when it is introduced into the molten aluminum at temperatures generally in excess of 1100 F. A major portion of the molten sodium rapidly bubbles to the surface of the aluminum melt and is lost into the atmosphere. The loss of sodium is not only costly, but can be extremely dangerous to personnel working in the vicinity.

The storage of the metallic sodium is also hazardous. Exposure to heat can cause an extremely hot, diflicult to extinguish sodium fire. To reduce the fire hazard and to minimize the oxidation of the metallic sodium, it is usually stored in essentially oxygen-free containers or under kerosene or naphtha.

A safe sodium-containing agent and less dangerous method of utilizing this agent to increase the sodium content of molten metals is greatly needed.

Once sodium has been added to molten metal, it is very difiicult to maintain sufficient sodium in the melt to effectively modify the silicon. At molten metal temperatures of from about 1200" to 1300 F. the sodium level often will only remain within a satisfactory concentration range for a period of from about 15 minutes to about one hour.

Materials are commercially available which inhibit the loss of sodium to the atmosphere after an initial addition of metallic sodium has been made to the metal. Generally these materials are in the form of a primarily nonmetallic block, which floats on the metal surface and gradually introduces sodium into the metal. Other materials melt and spread over the entire molten metal surface causing a barrier to form between the sodium-containing metal and the air. The molten barrier can include a sodium compound to gradually add sodium into the molten metal. Although these mixtures are adequate to maintain a satisfactory sodium level in the molten metal for extended periods, they are extremely inconvenient when casting the metal. For example, it is oftentimes desirable to cast metal from an open-ended crucible by 335,029 Patented Dec. 5, 1972 ladle transferring the metals from the crucible to a mold. Naturally a floating block or a molten layer over the liquid metal hinders this type of operation.

A modifying additive is greatly needed, which can add sodium and/or maintain the sodium level in molten aluminum without the inconveniences and hazards mentioned above.

It is, therefore, an object of this invention to provide a method to efiiciently add sodium to molten aluminum.

Another object of this invention is to provide a method of adding sodium to molten aluminum to facilitate the casting operation.

Other objects and advantages will become apparent during the course of the following description of the invention.

SUMMARY OF THE INVENTION Basically, the invention, which accomplishes the above objectives and minimizes the aforementioned detriments. is found in a process comprising introducing into molten aluminum, an essentially insoluble heavy metal containing sodium at a temperature exceeding the melting temperature of the sodium-containing heavy metal. After the heavy metal has melted, the sodium therein continues to gradually disperse into the aluminum. The sodium will continue to disperse within the aluminum and alloy therewith until the sodium has either been depleted from the heavy metal or an equilibrium condition has been achieved between the sodium content of the aluminum and heavy metal.

Suitable heavy metals for this process are essentially insoluble in the aluminum and readily alloyable with sodium. They are also characterized by a density greater than aluminum in the molten phase. Examples of such heavy metals are bismuth, cadmium, and lead and alloys based on these metals. An alloy of a heavy metal is herein defined as an alloy of bismuth, cadmium, or lead containing at least 50% of the base metal. Heavy metals are deemed to be essentially insoluble in aluminum when less than about 8% of the heavy metal will dissolve in molten aluminum at about the melting temperature of the aluminum. The amount of heavy metal dissolved in the aluminum can be determined by chemically analyzing a solidified sample of aluminum cast from a molten bath, which had been held for a sufficient time to settle undissolved heavy metal. The term aluminum is deemed to include alloys of aluminum containing at least 50% aluminum and chemically pure aluminum. As herein used an alloy is that defined in Grant, Hackhs Chemical Dictionary 28 (4th ed. 1968).

DESCRIPTION OF THE PREFERRED EMBODIMENT Lead being readily obtainable, inexpensive, and having a theoretical solubility in aluminum of about 1.5% at about 1220 F. is preferred for this process. More specifically, lead containing from about 0.1% to about 50% sodium is very effective in the process of this invention. Moreover, lead containing about 2% to about 20% sodium is even more beneficial to the instant process. Lead is easily alloyable with sodium; but it is essentially insoluble in aluminum.

This process is particularly useful in modifying silicon in aluminum alloys containing an amount of silicon up to about 15%. It is even more beneficial when the aluminum alloy contains from about 4% to about 13% silicon.

In operation, a heavy metal-sodium alloy can be added in either the liquid or solid form to an already molten bath of an aluminum. To avoid excessive loss of sodium by oxidation of the heavy metal-sodium alloy, it is preferred that the heavy metal modifying agent or master alloy be introduced into molten aluminum as a solid.

The heavy metal, having a higher density, readily sinks beneath the surface of the aluminum, thereby reducing the oxidation loss of the lead and sodium. Undissolved heaw metal settles to the bottom of the molten metal containing means, such as a crucible, where it remains until removal as a distinct phase.

While the molten master alloy remains positioned beneath the aluminum, sodium is believed to gradually diffuse from the heavier metal into the aluminum. As an equilibrium condition arises between the sodium content of the two metals, the rate of diffusion will diminish. As sodium is lost to the atmosphere from the air-metal interface of the aluminum, the diffusion of the sodium from the heavy metal into the aluminum replaces lost sodium. Through this process, a molten mass of aluminum having a substantially uniform sodium content can be maintained over an extended period of time.

In many aluminum casting operations, it is common to charge a solid ingot into a metal containing means, as a melting or holding crucible, to replenish the molten aluminum. If it is desired to minimize mixing of the aluminum and lead, during charging of solid ingot, numerous mechanical methods of keeping the metals separate are available. For example, stainless steel mesh or grating can be positioned Within the holding crucible at or above the interface of the substatically immiscible metals to minimize disturbance of the molten heavy metal when either an ingot or molten aluminum charge are put into the crucible.

The sodium content of the heavy metal can be replaced or rejuvenated by a number of methods. As an example, which can, optionally, also be employed to initially introduce sodium into the aluminum, by extending a pipe downwardly through the aluminum into the heavy metal, flushing the tube with inert gas, and charging small pieces of metallic sodium therethrough into the heavy metal in situ alloying the heavy metal and sodium will result. However, the metal in the crucible can be removed and the crucible recharged with aluminum and sodiumheavy metal alloy. Naturally, normal charging or immersion of the sodium-heavy metal alloy can be continued in lieu of the above described methods for maintaining sodium at desired levels.

The sodium can spread throughout the aluminum even though there are two separate and distinct layers of heavy metal and aluminum within the containing means. More rapid distribution of the sodium can be achieved by increasing the metal temperature or by agitating, such as stirring, the heavy metal and aluminum.

The claimed process is a more efficient method of alloying sodium with aluminum than by plunging metallic sodium directly into the liquid aluminum. It is believed this improvement is a result of the sodium in the heavy-metal alloy having a lower vapor pressure than substantially pure metallic sodium; consequently, the sodium is gradually released from the heavy-metal alloy and bubbling and loss of sodium to the atmosphere diminishes when the claimed method is employed. Optionally, however, metallic sodium can be added directly to the aluminum and the general sodium concentration achieved thereby can be maintained by use of the claimed invention.

The sodiumized aluminum can be cast into a shaped form by conventional modes of casting. Exemplary casting processes are centrifugal, die, investment, permanentmold, plaster-mold, and sand casting. As is known to those skilled in the art the degree of silicon modification in the microstructure of a solidified shaped cast article depends at least partially upon the rate of cooling from the molten phase and upon the amount of sodium retained in the alloy. Generally, as the rate of cooling decreases the amount of retained sodium in the aluminum must increase to afford satisfactory silicon modification. Satisfactory silicon modification is characterized by a breaking up or at least partial spheroidization of the acicular high silicon grains common to non-sodium modified aluminum-silicon alloys. This is easily observable by examination of the cast aluminum microstructure. An example of the microstructure of both a modified and an unmodified 14% silicon-aluminum alloy is shown in Doan, The Principles of Physical Metallurgy, 313 (3d ed. 1953 The amount of sodium-heavy metal master alloy, which must be melted to achieve silicon modification, is dependent upon the aluminum composition, casting method, and sodium content of the heavy-metal alloy. As the sodium in the heavy-metal alloy increases, the weight of heavy-metal alloy required diminishes and the sodium transfer rate from the heavy-metal alloy to the aluminum increases. The amount of master alloy necessary is at least that quantity sufficient to provide adequate sodium to satisfactorily modify at least a portion of the generally acicular silicon phase in the solid aluminum. Normally, satisfactory silicon modification can be achieved when less than about 0.1% sodium has been retained in the aluminum.

The following examples further illustrate the method of the claimed invention.

EXAMPLES 1-33 Lead alloy modifying agents were prepared by mixing various quantities of metallic sodium with molten lead and then casting and freezing the agent into smaller pieces.

A 300 gram sample of a molten 7% silicon-aluminum base alloy was maintained at a temperature of 1200 F. in a graphite crucible. The molten aluminum alloy was treated with 60 grams of a binary 4.5% sodium-lead alloy. The sodium-lead alloy settled to the bottom of the crucible and was held for two minutes before the aluminum was cast into a /1 inch diameter 'by 1 inch long shape in a graphite mold. Upon solidification of the casting, the aluminum alloy casting was examined metallographically and found to exhibit very fine spheroidal silicon particles common to a silicon-aluminum alloy having excellent modification.

The information in the following table further illustrates the effectiveness of the instant invention. Except where otherwise noted, the modification procedure was similar to that described above.

MODIFICATION OF 7% SILICON-ALUMINUM ALLOY CASTINGS Si modification* of dia. x 1" long, castings 1=Complete modification. Silicon particles spherical in shape and a size too small to resolve.

2= Essentially complete modification. Greater than about 75% of the silicon particles spherical in shape and a size less than about 0.0001 inch diameter.

3= Significant modification. About 25% to about 50% of the silicon particles spherical in shape and a size less than about 0.0001 inch diameter.

4 Poor modification. Less than about 10% of the silicon particles spherical in shape and a size less than about 0.0001 inch diameter.

Classification of silicon modification in the microstructure of solidified polished cast samples examined at a magnification of 500x.

For comparative purposes an aluminum casting alloy containing approximately 7% silicon was melted in a graphite crucible and heated to a temperature of 1400 F. The modified aluminum alloy was then poured into a graphite mold having a /1. inch diameter by 1 inch long cavity. After solidification, the casting was examined metallographically and found to exhibit elongated or acicular silicon particles. The microstructure was typical of an unmodified hypoeutectic aluminum-silicon alloy.

EXAMPLE 34 Approximately 300 grams of an aluminum-silicon alloy containing about 7% silicon was melted in a graphite crucible and raised to a temperature of 1350 F. Approximately 60 grams of a solid lead base alloy containing about 10% sodium was submerged in the molten aluminum to act as a modifying agent to refine the silicon structure in the solidified casting. After holding aluminum at 1350 F. for 1 hour and cooling the metal to 1200" F., it was poured into a sand mold having a cavity 1 inch by 1 inch by 2 inches. The metal was permitted to solidify normally within the sand mold. Examination of the cast microstructure revealed a good degree of silicon refinement; that is, greater than about 75% of the silicon particles in the specimen examined were generally spherical in shape and were of a size less than about 0.0001 inch in diameter.

The above examples illustrate that addition of a sodium containing lead base alloy to a molten aluminum-silicon alloy can supply sufiicient sodium to the aluminum to result in an adequate modification of silicon in the final cast product.

What is claimed is:

1. A method comprising introducing a sodium containing, essentially insoluble heavy metal selected from the group consisting of bismuth, cadmium, lead and alloys thereof into molten aluminum at a temperature exceeding the melting temperature of the sodium containing heavy metal.

2. The method of claim 1 wherein the aluminum contains silicon.

3. The method of claim 2 wherein the aluminum contains about 4% to about 13% silicon.

4. The method of claim 2 including the additional step of casting the molten aluminum into a shaped form.

5. The method of claim 4 including solidifying the light metal into a shaped cast article having a modified silicon microstructure.

6. The method of claim 1 wherein the heavy metal is lead.

7. The method of claim 6 wherein the sodium containing lead is solid when introduced into the aluminum.

8. The method of claim 6 wherein the lead contains from about 0.1% to about 50% sodium.

9. The method of claim 6 wherein the lead contains from about 2% to about 20% sodium.

10. The method of claim 1 including the additional step of casting the molten aluminum into a shaped form.

11. The method of claim 1 wherein metallic sodium is down charged through the molten aluminum into the heavy metal whereby the sodium content of the heavy metal is increased.

References Cited UNITED STATES PATENTS 3/1922 Edwards et al 75148 5/1927 Kraus et al 75--167 RICHARD O. DEAN, Primary Examiner Dated 5 December 1972 Patent No. 3 705 029 Inventor(s) George S. Foerster It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In column 2, line 1, delete "metals" and insert in its place --metal--.

-In column 4, the body of the table, second column from the left, line 5, delete "4-5" and insert in its place -4.5-.

In column 4, line 72, delete "modified" andinsert in its place -unmodified--.

Signed and sealed this 22nd day of May 19.7 3

(SEAL) Attest:

EDWARD M.FLETCHER,J R. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

